jschaefer7406 wrote: ↑
Tue. Jan. 09, 2018 9:40 am
We had been discussing this in a different thread, and just wanted to add to this.
My setup isn’t currently running an MPD. Have one, but haven’t installed it when replacing the flue pipe the last several times. Mine is a hand fired warm air furnace, and I do run a baro.
My initial question was regarding a draft change from .06wc to .04. I was running it at .06 for pea, and initially lowered it to .04 to burn nut.
Now, everyone says the lower draft pressure will “keep heat in the stove”. My experience is that, while the stack temp does indeed fall a bit (not much) when dropping .06 to .04, so does the plenum temp. So what I’m experiencing, is that reducing the flue draft doesn’t actually keep heat in the furnace, but only changes the range of heat. For example, I get the same heat at .06 as I did at .04, just with a different primary air setting.
So in essense, to me, changing the baro setting OR adjusting the primary air is essentially accomplishing the same thing, limiting the airflow (just on different sides of the fire).
Am I missing something?
A baro controls draft pressure more by lowering the exhaust temp and not by restricting the exhaust speed like an MPD does. Both will slow the exhaust, but after the baro to the pipe thimble, the pipe temp will be lower than if an MPD is used.
As for primary control verses MPD control, or no MPD control (Scotty's situation), I did an experiment yesterday after this topic came up. Here's the numbers taken with an IR gun and a Dwyer mark II mano.
3:50 pm 32F OAT.
Primary dampers open 1/8 inch, all. MPD almost fully closed (normal settings for 32 OAT).
Top plates over firebox 680 and 715.
Pipe below MPD 129
Pipe three feet after MPD 116
3.55 pm MPD fully open, no other changes.
Plates both 800+ (off the scale of my IR gun).
Pipe before MPD 160
Pipe 3 ft after MPD 138
4:05pm closed primary dampers half way to 1/16 inch all. MPD still fully open.
4:10 pm Plates 800+ but not showing dull red (means they are still below 1000F).
Pipe before MPD 170
Pipe 3 ft after MPD 143
Temps/mano still climbing, discontinued test and reset dampers to usual positions.
So, even though I closed the primary halfway, with affectively no MPD help, the temps and the mano readings continued to climb, verses when it had more primary opening and an almost fully closed MPD at the start.
From past experience, when I've forgotten to close the MPD,
the stove temps would level off below the point that the top plates would turn dull red (1000 F), but the pipe chimney temps would continue to rise more causing even stronger draft (faster exhaust flow). And the kitchen would not get as much warmer as you might expect. However, the big sign of wasted heat is that the fuel in the firebed would only last about half as long.